Localization of alphoid human satellite DNAs using fluorescence in situ hybridization (FISH) of metaphase chromosomes following treatment with 5-azacytidine to produce undercondensation showed that human α-satellite DNA is not sensitive to the condensation-inhibition effect of 5-azacytidine. The difference in heterochromatic DNA subsets was particularly evident in the constitutive heterochromatin of chromosomes 1 and 9. Comparison of the results obtained after FISH with those obtained from electron microscopy and G-banding enabled accurate localization of this DNA domain